Since magnetic north is slightly shifted from true north, true north cannot be measured using a magnetic compass. However, administrative maps are produced based on true north, and the Construction Standards Act is also based on true north. Therefore, in the field of civil engineering and construction, true north must be determined correctly. In underground tunnel construction, in particular, a magnetic compass does not function correctly because of the effect of a mineral vein or the like.
There is a known conventional gyro compass, as a true north detector that detects true north correctly, which detects the earth's rotational angular velocity to determine true north. As disclosed by JP 2002-296037 A, U.S. Pat. Nos. 6,594,911 and 6,918,186, gyro compasses are generally orthogonal three-axis type devices. These gyro compasses are large in size and costly to manufacture.
A number of one-axis or two-axis type devices have been suggested in order to reduce the size and cost. An azimuth indicator disclosed by JP 6-3149 A is a single-axis type device that has an angular velocity sensor (gyro sensor) rotated on a horizontal base at a fixed angular velocity. An azimuth indicator disclosed by JP 6-11350 A is a single-axis type device and has a gyro sensor rotated on a horizontal base to obtain provisional true north. True north is obtained by obtaining a bias based on a measurement value of an angular velocity in the provisional true north and separately input latitude. A true bearing measuring device disclosed by JP 11-160072 A is a two-axis type that does not need a horizontal plane and has a gyro sensor rotated for every 90° or 120° on two rotation bases to remove a bias. An azimuth indicator disclosed by JP 11-190633 A is a single-axis type device that does not need a horizontal plane and has a gyro sensor and an acceleration sensor indicate three directions on a rotating base. A latitude however must be input in this device. An azimuth detector disclosed by JP 2001-215121 A is a single-axis type device that has a gyro sensor rotated on a horizontal plane to obtain true north based on a sinusoidal gyro output.
Most of such gyro compasses have a gyro sensor and an acceleration sensor rotated on a rotation base, and a large space must be secured for its large rotation angle, which limits how compact the device can be. Most single-axis type gyro compasses need a horizontal plane, which make it cumbersome to handle them. Single-axis type gyro compasses that do not need a horizontal plane have been suggested, but their precision about azimuth finding is inferior to those of three-axis type compasses.
Therefore, the applicant has suggested a single-axis type azimuth measuring device that does not need a horizontal plane in the disclosure of JP 2008-215956 A. The azimuth measuring device disclosed by the document takes into account a UVW rectangular coordinate system in addition to an XYZ rectangular coordinate system. Six directions, +U, −U, +V, −V, +W, and −W are provided apart at intervals of 60° when they are projected orthogonally on an YZ plane. Elevation angles formed between the U, V, and W axes and the YZ plane are each 35.26°. The azimuth measuring device includes a rotational angular velocity sensor that detects rotational angular velocities ωU, ωV, and ωW around the respective axes in the UVW rectangular coordinate system, a gravitational acceleration sensor that detects gravitational accelerations gU, gV, and gW in the respective axial directions, a first stepping motor that rotates the rotational angular velocity sensor and the gravitational acceleration sensor 60×n° (n: a natural number) around the X-axis for positioning, and a second stepping motor that swings the rotational angular velocity sensor and the gravitational acceleration sensor ±35.26° around an axis orthogonal to the X-axis for positioning. The azimuth measuring device measures the rotational angular velocities ωU, ωV, and ωW and the gravitational accelerations gU, gV and gW around the respective axes and coordinate-transforms the obtained rotational angular velocities ωU, ωV, and ωW and the gravitational accelerations gU, gV, and gW to produce ωX, ωY, and ωZ and the gravitational accelerations gX, gY, and gZ in the XYZ rectangular coordinate system. Then, an azimuth angle Ψ is calculated based on the obtained rotational angular velocities ωX, ωY, and ωZ and the gravitational accelerations gX, gY, and gZ. Using the azimuth measuring device, the rotational angular velocity sensor and the gravitational acceleration sensor are swung only ±35.26°, so that the rotation angle is small and a necessary space is not larger than the space required by the conventional azimuth measuring device. Therefore, the size can be reduced as compared to the conventional single-axis azimuth measuring device.